Skid wire for pipe type electric cables

ABSTRACT

A pipe type electric cable in which the core comprising a central conductor surrounded by insulation which, in turn, is surrounded by a conductive screen, is received within a fluidtight metal tube and is separated from and conductively connected to the inner surface of the pipe by one or more skid wires wound around the screen, each of which skid wires comprises one or more metal cords extending longitudinally of the skid wire and surrounded by a semi-conductive synthetic resin which prevents contact of the cords with the inner surface of the pipe.

United States Patent 1 Priaroggia SKID WIRE FOR PIPE TYPE ELECTRICCABLES [75] Inventor: Paolo Gazzana Priaroggia Milan,

Italy [73] Assignee: Industrie Pirelli Societa per Azioni,

Milan, Italy [22] Filed: Sept. 16, 1974 [21] Appl. No.: 505,964

[30] Foreign Application Priority Data Oct 12, 1973 Italy 30035/73 [52]US. Cl. 174/10; 174/25 R [51] Int. C1. l-IOlB 7/22; H01B 9/06 [58] Fieldof Search 174/10, 24, 25 R, 25 G, 174/26 R, 26 G, 27, 108, 135, 136

[56] References Cited UNITED STATES PATENTS 2,516,747 7/1950 Bennett174/25 R 2,615,074 10/1952 Bronovicki 174/10 2,665,328 l/l954 Atkinsonet al 174/25 R 1 3,047,652 7/1962 Ege 174/25 R X Oct. 28, 1975 3,080,4463/1963 Volk 174/25 R 3,579,863 5/1971 McGrath 174/10 3,594,492 7/1971Bahder et a1. 174/108 X 3,621,110 11/1971 McGrath..- 174/10 3,673,3076/1972 Eager, Jr. et al 174/25 R FOREIGN PATENTS OR APPLICATIONS1,151,413 l/1958 France 174/25 R Primary Examiner-Arthur T. GrimleyAttorney, Agent, or Firm-Brooks I-Iaidt I-Iaffner & DeLaHunty [57]ABSTRACT A pipe type electric cable in which the core comprising acentral conductor surrounded by insulation which, in turn, is surroundedby a conductive screen, is received within a fluid-tight metal tube andis separated from and conductively connected to the inner surface of thepipe by one or more skid wires wound around the screen, each of whichskid wires comprises one or more metal cords extending longitudinally ofthe skid wire and surrounded by a semi-conductivesynthetic resin whichprevents contact of the cords with the inner surface of the pipe.

8 Claims, 3 Drawing Figures U.S. Patent Oct. 28, 1975 1 SKID WIRE FORPIPE TYPE ELECTRIC CABLES The present invention relates to pipe typeelectric cables, especially for high voltages, which comprise a coreincluding a central conductor surrounded by one or more layers ofinsulation and an outer electrostatic screen, a helically wound wire orwires, known in the art as skid wires, and surrounding the electrostaticscreen a fluid-tight, rigid metal tube or pipe loosely receiving thecore with the skid wires therearound. Such cables do not-have a sheathintegral with, or tightly surrounding, the insulation or electrostaticscreen of the core.

The skid wires serve, during the laying or installation operations, aswell as during the thermal cycles, to mechanically protect the cablecore. In addition, they assist in reducing the possibility of abrasionof the inner surface of the pipe during installation, and theyconductively interconnect the electrostatic screen of the core with thepipe.

Up to the present time, four main types of skid wires were known,namely:

1. Bare metal skid wires of good conductivity and preferably made ofcopper, bronze, brass, zinc, stainless steel or aluminum.

2. Skid wires of dielectric or insulating material, such as a highlyresistant synthetic resin, usually linear polyethylene or nylon. A

3. Skid wires of dielectric or insulating material with helically wound,exterior metal tapes.

4. Composite skid wires of insulating material with metal conductorsembedded therein and extending from surface to surface thereof.

As is known, the metal skid wires set forth in (l) hereinbefore have ahigh friction coefficient. Therefore, they cause substantial abrasion ofthe inner surface of the pipe, and this causes problems, for example, inthe use of aluminum pipes as the outer pipe. The employment of aluminumpipe would be of substantial interest because, by using such pipes, thelosses due to eddy currents, with the same core, would be much lowerthan with iron tubes and, therefore, a greater current carrying capacitywould be obtained.

In addition, such metal skid wires limit the maximum core length whichcan be drawn in a pipe, when it is taken into account that the wholedrawing strain is transferred to the core conductor.

Dielectric skid wires of the type described in (2) hereinbefore find useonly in very special cases, but not with metal pipes, because they donot provide the necessary electrical interconnection of theelectrostatic screen of the core with the pipe. Such dielectric skidwires have, moreover, the disadvantage that they do not offer asufficient mechanical resistance to the longitudinal stresses occurringduring drawing and, therefore, they are subjected to rupturing and toconsequent pulling out of the broken portions.

Dielectric skid wires of the type described in (3) hereinbeforeobviously have a greater friction coefficient than polyethylene or nylonand, therefore, there is always the risk of undesirable abrasion of theinner surface of the pipe.

Although skid wires of the type described in (4) hereinbefore providethenecessary electrical interconnection between the screen of the coreand the pipe, the metal parts extending from the surface of the skidwire cause the undesirable effects of a high friction coefficient on theinner surface of the pipe. Moreover, said composite skid wires aredifficult to manufacture, because of the difficulties encountered infinding a resin of low friction coefficient which is able to firmlyadhere to the metal.

One object of the present invention is that of eliminating thedisadvantages encountered with cables comprising conventional skid wiresand at the same time, to provide cables comprising skid wires which maybe easily manufactured.

According to the invention, the core of the cable is provided with skidwires which have a low friction coefficient, which provide very goodelectrical contact between the electrostatic screen of the core and thepipe, and which provide substantial resistance to the longitudinaldrawing strains which occur during the installation of the core in thepipe.

More particularly, the object of the present invention is an electriccable of the pipe type, provided with one or more skid wires helicallywound up on the electrostatic screen of the core, characterized in thateach skid wire is constituted by an elongated element of synthetic resinmade semi-conductive by means of an appropriate filler and one or moremetallic cords surrounded by the resin and extending for the full lengthof said element.

Such object and other objects of the invention will be apparent from thefollowing detailed description of a presently. preferred embodiment ofthe invention, which description should beconsidered in conjunction withthe accompanying drawing in which:

FIG. 1 is a perspective view, partly in section, of a pipe cableconstructed in accordance with the invention;

FIG. 2 shows, in enlarged scale, a perspective view of aportion of theskid wire of the cable shown in FIG. 1; and

FIG. 3 is a longitudinal section of the skid wire shown in FIGS. 1 and 2and is taken along the line 3-3 indicated in FIG. 2.

FIG. 1 illustrates a screened cable core 10 laid in a pipe ll. The core10 comprises a central stranded conductor l2, surrounded by a firstconductive screen 13 which, in turn, is surrounded by layers ofinsulation 14.

The insulation 14 is surrounded by a second conductive, electrostaticscreen 15. The screen 15 is of the conventional type, namely, isconstituted by a metallic tape wound helically around the insulation 14.

Two skid wires 16 and 17 are wound helically around the electrostaticscreen 15. Although two skid wires 16 and 17 have been shown, it will beapparent to those skilled in the art that the number of the skid wiresapplied can be different, for example, greater than two or only one skidwire. In case only one skid wire is used, said skid wire, for a givensize of core 10, preferably will be applied at a pitch corresponding toone half of the pitch normally employed when two skid wires are used.

As illustrated in FIG. 2, each skid wire 16 and 17 is constituted by atape or elongated element 18 of synthetic resin which is madesemi-conductive by means of appropriate fillers. Preferably, thesynthetic resin is high density polyethylene (friction coefficient onmetal 0.25), polyvinyl chloride (friction coefiicient on metal 0.36) ornylon (friction coefiicient on metal 0.l9 0.20). The filler used to makethe synthetic resin semiconductive preferably is carbon black.

Three metal cords 19, 20 and 21,- for example, of stranded wires andpreferably made of steel, extend longitudinallyof the element 18 and arecompletely embedded in, or surrounded by, the resin of the element 18 sothat the cords 19, 20 and 21 cannot engage the inner surface of the pipe11. Although steel is preferred asthe metal for the cords 19, 20 and 21,because of its strength, other metals may be used. The cords 19, 20 and21 run parallel to each other and are spaced apart from one another fromone end of the skid wire to the other. The number of cords may bedifferent from three and, for example, only one steel cord or more thanthree cords could be used.

Each skid wire 16 and 17 can have the shape indicated in FIG. 2, namely,can be D-shaped in crosssection. Thus, the skid wire 16 has a surface 22of semiconductive resin which contacts the electrostatic screen 15 ofthe core 10. Said surface 22 is substantially flat but, if desired, canbeconcave which permits it to adhere better to the electrostatic screen15. The

lower surface 23 of the skid wire 16, also of semiconductive syntheticresin, is rounded so as to provide the conventional D-shape, but itcould have another appropriate shape. For example, the surface 23 couldbe a flat surface, and, if desired, the skid ,wire 16 could rectangularin cross-section. Moreover, the skid wires 16 and 17 can be the same ordifferent, provided that their maximum thickness s (FlGQ3) is the same.

The production of the skid wires 16 and 17 according to the invention isvery easy, since it consists in the mere extrusion of the syntheticresin 18, chosen as desired and appropriately loaded, on the steel cordor cords 19, 20 and 21 to be embedded.

Owing to the low friction coefficient of such skid wires 16 and 17, atthe most preferably about 0.20 (dynamic friction coefficient on metals),with respect to the friction coefficient of "the skid wires of the priorart, which, as an average ranges from 0.5 to 0.6, the core can be easilydrawn into the pipe 11 during installation, and can freely slide in thepipe 11 during thermal cycles, without causing significant abrasion ofthe inner surface 23 of the pipe 11.

In addition, the semi-conductive characteristic of the loaded syntheticresin of the elongated element 18 ensures a good conductive connection'between the screen and the pipe 11; The resistivity of the resin shouldbe at least 1000 ohms/m. and, preferably, is in the range froml000-l0,000 ohms/m.

The cords l9, 20'and 21 embedded-in theresin element l8 cooperate withthe latter by taking the longitudinal stresses exerted 'on the skidwires16 and 17 during drawing in operations for laying the core 10 inthe P p Although a preferred embodiment of the present invention hasbeen described and illustrated, it will be understood by those skilled'in the art that various modifications may be made without departingfrom the principles of the invention,

What is claimed is:

1. An electric cable member for a pipe type cable comprising an electricconductor surrounded by insulation, a conductive screen around saidinsulation, and at least one skid wire wound around and in contact withsaid screen, said skid wire comprising at least one metal wire extendinglongitudinally of said skid wire for the length thereof and surroundedby a semi-conductive synthetic resin.

2. An electric cable member as set forth in claim 1, wherein saidsemi-conductive synthetic resin is a resin selected from the groupconsisting of high density polyethylene, polyvinyl chloride and nylonloaded with carbon black.

3. An electric cable member as set forth in claim 2, wherein saidsemi-conductive synthetic resin has a resistivity in the range from 1000to 10,000 ohms per meter.

, 4. An electric cable member as set forth in claim 1, wherein saidmetal wire is stranded.

5. An electric cable member as set forth in claim 4, wherein the metalof said metal wire is steel.

6. An electric cable member as set forth in claim 1,

wherein said skid wire comprises at least three stranded extendinglongitudinally of said skid wire for the length thereof and surroundedby a semi-conductive synthetic resin.

1. An electric cable member for a pipe type cable comprising an electricconductor surrounded by insulation, a conductive screen around saidinsulation, and at least one skid wire wound around and in contact withsaid screen, said skid wire comprising at least one metal wire extendinglongitudinally of said skid wire for the length thereof and surroundedby a semi-conductive synthetic resin.
 2. An electric cable member as setforth in claim 1, wherein said semi-conductive synthetic resin is aresin selected from the group consisting of high density polyethylene,polyvinyl chloride and nylon loaded with carbon black.
 3. An electriccable member as set forth in claim 2, wherein said semi-conductivesynthetic resin has a resistivity in the range from 1000 to 10,000 ohmsper meter.
 4. An electric cable member as set forth in claim 1, whereinsaid metal wire is stranded.
 5. An electric cable member as set forth inclaim 4, wherein the metal of said metal wire is steel.
 6. An electriccable member as set forth in claim 1, wherein said skid wire comprisesat least three stranded metal wires.
 7. An electric cable member as setforth in claim 6, wherein said metal wires are parallel to and spacedfrom each other.
 8. A pipe cable comprising a metal pipe, a cable corewithin said pipe, said core comprising an electric conductor surroundedby insulation and a conductive screen around said insulation, and atleast one skid wire wound around said screen and intermediate it andsaid pipe, said skid wire comprising at least one metal wire extendinglongitudinally of said skid wire for the length thereof and surroundedby a semi-conductive synthetic resin.